1. Field of the Invention
The present invention relates to a laser oscillator, and relates in particular to the extension of the service lives of parts and members in a case, and to the limiting of the deterioration of a laser gas.
2. Description of the Related Art
A conventional carbon dioxide laser apparatus will now be described while referring to FIG. 13. FIG. 13 is a schematic diagram illustrating a conventional carbon dioxide laser apparatus. In FIG. 13 is shown a carbon dioxide laser apparatus, comprising a laser oscillator 1, a discharge electrode 2, a partial reflector 4, a full reflector 5, a heat exchanger 6 and a power supply board 8, in which devices are stored that precipitate a discharge at the discharge electrode 2, that control a gas circulation blower 3, and that set up the laser oscillator 1 in a vacuum state. A cooling unit 7 cools the partial reflector 4, the full reflector 5 and the heat exchanger 6 by conveying cooling water thereto, and a controller 9 controls the operation of the laser oscillator 1. A laser gas 10 fills the interior of the oscillator 1, and a laser beam 11 is emitted by the laser oscillator 1.
The operation of the laser oscillator 1 in FIG. 13 will now be explained. First, when a start signal is transmitted by the controller 9 to the power supply board 8, the gas circulation blower 3 is rotated, and the laser gas 10, which for the carbon dioxide laser apparatus consists of a mixture of CO2, N2 and He that fills the interior, is circulated throughout the laser oscillator 1. Then, when in this state an output signal is received from the controller 9, a high voltage that is applied to the discharge electrode 2 excites molecules in the laser gas 10. The molecules N2, which are excited by the discharge, hold kinetic energy, and initiate active movement within the laser gas 10 by colliding with CO2 molecules, to which the kinetic energy they hold is transferred. The CO2 molecules, after a specific time has elapsed following the transferral to them of the kinetic energy, emit light and then fall to a stable ground level. The quantity of light emitted depends on the quantity of the CO2 and the N2 molecules that are excited by the discharge. The emitted light is reflected and amplified between the partial reflector 4 and the full reflector 5. Since a part of the light is extracted by the partial reflector 4, and the remaining light is further reflected and returned by the full reflector 5, the reflection and amplification process is repeatedly performed.
A quantity of the laser beam 11 equivalent to the output instructed by the controller 9 is emitted externally. Since in the arrangement in FIG. 13 the direction in which the laser beam 11 is emitted, the direction in which a discharge occurs, and the direction in which the laser gas 10 passes through the discharge electrode 2 are perpendicular to each other, this is called a three-axis orthogonal type. For this, the extracted laser beam 11 is focused on a machine or a measurement device (not shown) to perform a machining process or to obtain a measurement.
The internal detailed structure of the laser oscillator 1 in FIG. 13 is shown in FIG. 14. A box 13 is a sealed container that is filled with the laser gas 10, but at a concentration, since a stable discharge is required in order to obtain a stable laser output, that is less than one atmosphere. Windows, on both sides of the box 13, are provided to facilitate the insertion and removal of internal parts for production or for maintenance, and doors 14 are attached to close and seal these windows. Screws are employed to secure the doors 14 to the box 13, and O rings (not shown) are used to provide airtight seals for them.
The main parts inside the box 13 are the discharge electrodes 2, the gas circulation blower 3, the partial reflector 4 and the full reflector 5 (not shown), and the heat exchanger 6, which have been explained while referring to FIG. 13. Each of the discharge electrodes 2 includes an electrode tube 15 and an insulator 16, and of these the electrode tube 15 is constituted by a water channel 15a, along which cooling water is supplied to the interior, a metal tube 15b for covering the water path 15a and a dielectric 15c for covering the metal tube 15b. A material such as glass, having a higher permittivity than the insulator 16, is employed as the dielectric 15c. Thus, when a high frequency voltage is applied by the power supply board (not shown in FIG. 14) to the two electrode tubes 15, which are separately arranged at a specific distance from each other, a very smooth and stable discharge can be generated through the dielectric 15c. In order to prevent a discharge from other than the opposed, facing portions of the electrode tubes 15, the opposed, facing faces of the electrode tubes 15 are partially exposed, while the other portions are covered with the insulator 16. In addition, insulating joints 17 and pipes 18 are also provided for cooling the electrode tubes 15. And to cool the laser gas 10, which passes between the discharge electrodes 2, it is introduced, along a gas duct 25, to the heat exchanger 6.
For the carbon dioxide laser oscillator to generate the laser beam 11, a discharge occurs between the two electrode tubes 15. By means of the discharge, not only is the N2 excited, but also heat and ultraviolet rays are generated. As is shown in FIG. 15, the wavelength of the emission spectrum produced by the discharge is distributed mainly between 280 nm and 450 nm, and an emission of ultraviolet rays equal to or less than 300 nm was observed. Normally, most light having a wavelength of 300 nm or less is absorbed by the atmosphere, and a transmittance of only about 1% is acquired. However, as previously mentioned, since the carbon dioxide laser is activated in a state wherein a pressure considerably less than one atmosphere is maintained, generally wherein the pressure ranges from 1/10 to ⅓ of an atmosphere, a high transmittance is obtained for a wavelength of 300 nm.
When a laser beam is generated by a conventional laser oscillator, ultraviolet rays are produced by a discharge. For this, the laser oscillator 1 employs as a laser gas a mixture of CO2, N2 and He, and when a discharge occurs, CO2 is decomposed and NOX is generated.2CO2+N2→2CO+O2+N2→2CO+2NO.
It should be noted that NO in the chemical formula may further react with oxygen (O) and be changed to NO2.
Due to the generation of NOX, the oxygen in the laser gas in the box 13 is consumed, thereby reducing the quantity of CO2. Thus, for a sealed laser oscillator 1 a problem that arises is a reduction in the strength of the laser that is output. For a conventional sealed laser oscillator 1, for which the laser gas 10 is employed inside a sealed container, the box 13, the length of time the laser oscillator 1 can be continuously operated in the sealed state is severely limited, due generally to the deterioration suffered by the laser gas 10, i.e., the reduction in the CO2 and a slight leakage of the laser gas 10 from the container 13.
Thus, the laser gas 10 in the box 13 must be changed frequently, a situation that contributes to the untoward wasting of the laser gas 10. And inside the box 13, the ultraviolet rays that are generated are scattered and reflected by interior faces, such as those of the doors 14, so that they strike the insulators 16, the joints 17 and the O rings of the discharge electrodes 2, accelerating the deterioration and reducing the useful life of these and other laser oscillator 1 parts, and necessitating the frequent performance of maintenance.
The above problem corresponds to a problem described, for example, in JP-A-5-136506 and JP-A-4-100284 related to the deterioration by the laser gas and ultraviolet rays of the components in a container. In these publications, the excimer laser oscillator is disclosed, and deterioration of the parts is prevented by constructing them of a material such as fluorocarbon resin or ceramics. However, for the carbon dioxide laser oscillator described in these specifications, the gas pressure in the box 13 tends to be less than one atmosphere, and in such a case, the transmittance of ultraviolet rays having a wavelength equal to or smaller than 300 nm is increased, even when parts (members) are made of a fluorocarbon material. Thus, it is apparent that deterioration is accelerated when the internal members are irradiated by the ultraviolet rays.
When vinyl chloride (C—Cl) is used, molecular binding is severed by a light having a wavelength that is equal to or less than 360 nm, while the molecular binding of polyester (C—O) is severed by a light having a wavelength equal to or less than 340 nm, and the molecular binding of a fluorocarbon resin (C—F, C—H) is severed by a light having a wavelength equal to or less than 300 nm. When fluorocarbon resin is decomposed and fluorine (F) is released into the box 13, normally, the optimal dew point for laser oscillation is −40° C. to −60° C. and a slight trace of water must be present. But it has been found that when fluorine binds to the water H2O present in the laser gas 10, very corrosive hydrogen fluoride (HF) is generated. Then, according to the following mechanism, the hydrogen fluoride damages the glass dielectric 15c and the silicon insulator 16 that are used for the discharge electrodes 2 of the laser oscillator 1.4HF+SiO2→SiF4+2H2O.
Since the joint 17 is connected to the discharge electrodes 2 to which a high voltage is applied at a high frequency, the joint 17 must be composed of a resin having a low transmittance, and a ceramic having a high transmittance is not appropriate because a creepage discharge is generated. Further, fluoride rubber, such as viton, is used for the O ring because the deterioration of the O ring is accelerated under these conditions. And of the resins, a fluoride resin has proven to be the most resistant when used in places irradiated by ultraviolet rays of 300 nm. Since no other materials have been found to be more appropriate than those presently in use, the current technique can not prevent the discharge of hydrogen fluoride.
In addition, a system for protecting the discharge electrodes using a nonorganic insulator, such as glass, is described in JP-A-10-303483. In an environment in which hydrogen fluoride is present, glass is damaged, and a dust consisting of SiF4 is discharged inside the sealed container, or the dielectric made of glass can not be protected, so that the life of the discharge tube is reduced.